DH-485 Industrial Local Area Network

DH-485, standing for Data Highway 485, is an industrial local area network (LAN) protocol used primarily in Allen-Bradley (Rockwell Automation) PLC systems. This protocol facilitates communication among various devices in industrial control systems. Here are some key aspects of DH-485:

1. Purpose and Use: DH-485 is designed for industrial environments to enable communication between Allen-Bradley PLCs, computers, PanelView terminals, and other industrial control devices. It’s particularly used in small to medium-scale control applications.
2. Based on RS-485 Standard: The “485” in DH-485 refers to the RS-485 communication standard, which is a widely used method for serial data transmission. RS-485 supports multi-point systems, making it suitable for creating networks of multiple devices.
3. Network Configuration: DH-485 allows for the creation of a network where multiple devices can communicate with each other. It supports up to 32 devices on a single network, making it a viable option for smaller industrial control systems.
4. Communication Speed and Distance: DH-485 typically operates at speeds up to 19.2 Kbps. While this is relatively slow compared to modern standards, it is sufficient for many industrial applications. The network can span distances up to 4000 feet (approximately 1200 meters), depending on the network configuration and cable quality.
5. Topology: The network topology for DH-485 is usually linear or star. This means devices are either connected in a series (linear) or branched off from a central point (star).
6. Applications: DH-485 is primarily used in automation applications involving Allen-Bradley PLCs, like the SLC 500 series. It’s suitable for tasks such as PLC programming, HMI interfacing, and data collection.
7. Ease of Implementation: One of the advantages of DH-485 is its relative simplicity and ease of implementation compared to more complex network protocols. This makes it an accessible option for many industrial settings.
8. Legacy Protocol: Similar to DF1, DH-485 is considered a legacy protocol in the context of modern industrial communication standards. Newer systems often use Ethernet-based protocols, but DH-485 is still found in many existing installations due to its reliability and the widespread use of Allen-Bradley equipment.

In summary, DH-485 is a robust, tried-and-tested industrial network protocol that has been used effectively in various Allen-Bradley PLC systems for communication in smaller industrial automation environments. Despite the evolution of faster and more advanced communication technologies, DH-485 remains relevant in specific applications and existing installations.